Modeling the reversible addition-fragmentation chain transfer process in cumyl dithiobenzoate-mediated styrene homopolymerizations: Assessing rate coefficients for the addition-fragmentation equilibrium

A full kinetic scheme for the free-radical reversible addition-fragmentation chain transfer (RAFT) process is presented and implemented into the program package PREDICI (R). With the cumyl dithiobenzoate-mediated bulk polymerization of styrene at 60 degreesC as an example, the rate coefficients associated with the addition-fragmentation equilibrium are deduced by the careful modeling of the time-dependent evolution of experimental molecular weight distributions. The rate coefficient for the addition reaction of a free macroradical to a polymeric RAFT species (k(beta)) is approximately 5 . 10(5) L mol(-1) s(-1), whereas the fragmentation rate coefficient of the formed macroradical RAFT species is close to 3 . 10(-2) s(-1). These values give an equilibrium constant of K = k(beta)/k(-beta) = 1.6 . 10(7) L mol(-1). Conclusive evidence is given that the equilibrium lies well on the side of the macroradical RAFT species. The high value of k(beta) is comparable in size to the propagation rate coefficients reported for acrylates. The transfer rate coefficient to cumyl dithiobenzoate is close to 3.5 . 10(5) L mol(-1) s(-1). A careful sensitivity analysis was performed, which indicated that the reported rate coefficients are accurate to a factor of 2. (C) 2001 John Wiley & Sons, Inc.